The present invention relates to a closure or termination structure for electric gas discharge tubes or lamps. Such tubes are formed with bulbs made of ceramic and sapphire materials. The present invention also relates to the lead construction i.e. to the method and structure by which the current is brought into the tube which causes it to glow.
It is well known in the art that gas discharge tubes or lamps may be made of ceramic or sapphire and that such lamps may have alkali metals disposed inside the tube. Such alkali metal vapor lamps have several operational problems because they operate at high temperatures and because it is desirable and common to use the closure member to serve as the current lead-in structure. As a result only metals can be used for closure which are chemically resistant to alkali metal vapors and which have a coefficient of thermal expansion which is similar to that of the ceramic or sapphire materials used in the tube.
If the ceramic tube or lamp is made of a poly-crystalline alumina, niobium can be used for the closure member. Although niobium's coefficient of thermal expansion is slightly less than that of the ceramic the difference in thermal coefficients may be compensated for if the closure member is made of a thin plate the surface of which is roughened by sintering a foreign material thereto. This sintered structure of niobium and foreign material has a greater elasticity and better adhesive strength in conjunction with the solder than the niobium alone. Such a structure is taught in U.S. Pat. No. 3,243,635.
It is also known in the art that a better bond can be formed using a ceramic tube if the ceramic tube is closed by a ceramic plug having a metallized surface. The reason for this is that the plug or end cap can be made of the same material as the ceramic tube therefore the coefficient of thermal expansion and the alkali resistant properties are obviously well matched. This structure is taught in U.S. Pat. No. 3,693,007.
Even where end caps and bulbs having identical coefficients of thermal expansion are soldered together, stresses may occur if the masses of the components are different and the heating and cooling does not take place simultaneously. Naturally these effects are common ones since the masses of the end caps and the tubes are not the same nor is their proximity to the heating elements identical, nor is the heat flow away from the tubes and caps identical since that would depend on the physical disposition or placement of the members. Therefore when lamps are switched on or off or even in some steady-state conditions it is common that different thermally generated mechanical stresses will exist.
The end closure is particularly less certain when sapphire tubes are used instead of poly-crystalline alumina ceramic tubes. The coefficient of thermal expansion of the sapphire not only is larger than that of the poly-crystalline alumina but it is also larger than that of niobium and the ceramic plugs respectively.
It is an object of the present invention to teach an end cap or closure construction for ceramic or sapphire tubes which is more resistant to thermally caused stresses and which does not require an increase in the number of soldering and other closing or sealing operations.